Attention has been drawn to gallium nitride (GaN) as a material for semiconductor elements that emit blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disks and displays, and a light-emitting diode (LED) that emits blue light is used for displays, lighting and the like. Further, an ultraviolet LD is expected to be used in the field of biotechnology or the like, and an ultraviolet LED is expected to be used in fluorescent lamps to emit ultraviolet light.
Generally, GaN crystals that are used for LDs or LEDs are formed by heteroepitaxially growing GaN crystals on a sapphire substrate using the vapor phase epitaxial method. GaN crystals obtained in this manner generally have a dislocation density of 108 cm−2 to 109 cm−2 and thus are of inferior quality. As a solution to this problem, for example, an ELOG (epitaxial lateral overgrowth) method has been developed. By this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2. This method, however, includes complicated processes and thus is disadvantageous.
Meanwhile, besides the vapor phase epitaxy, a method of growing crystals from a liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of GaN crystals, conventionally, conditions as harsh as 8000 atm (8000×1.013×105 Pa) at 1200° C. are required for growing GaN crystals from a liquid phase. In order to solve this problem, a method using a Na flux has been developed. This method allows GaN crystals to be obtained using relatively mild conditions. Moreover, there also is a method in which a mixture of Ga and Na is melted by pressurizing and heating under an atmosphere of nitrogen gas containing ammonia, and a melt thus obtained is used to grow crystals for 96 hours, so that single crystals having a maximum crystal size of about 1.2 mm are obtained (see, for example, Patent document 1).
Furthermore, a method has been proposed in which a reaction vessel and a crystal growth vessel are formed as separate units so as to suppress natural nucleus development, and thus crystals are grown to be large (see, for example, Patent document 2).
However, in the field of Group-III-element nitrides such as GaN, there has been a demand for further improvements in growth rate and quality.    Patent document 1: JP 2002-293696 A    Patent document 2: JP 2003-300798 A